MODES OF INTERACTION WITH INPUT DEVICES

REQUEST MODE: the transfer is initiated by the program, but the transfer is triggered by the device.
-- E.g., read from keyboard statements in most computer languages, or scanf in C language. The trigger is the pressing of special keys such as ENTER or RETURN keys.
-- There be an indefinite time lapse between initiation and transfer (usually, the program hangs up during this gap).

SAMPLE MODE: the initiation and trigger both originates from the program.
-- E.g., most locator devices can be sampled at any time.
-- Of course, not all devices can be accessed in this mode.

EVENT MODE: the device initiates and triggers the transfer. May also be called the "interrupt mode".
-- This is sometimes implemented to causes the measure and other data (e.g., device ID) to be placed in an event queue. But now, the program has to busy wait for events.
-- To avoid busy waiting, the GLUT approach is for the program to specify callback functions for classes of events. When an event occurs, the associated callback function is invoked at once.